Isabel Marant appears suddenly from around a corner looking slightly harried and wrestling herself into a huge black cable-knit sweater that immediately looks like the only thing one should be wearing today.

The French fashion designer's Parisian headquarters is opposite the Banque de France. In amongst the grand,establishment façades of the premier arrondissement it has a discreet, dark, industrial-style doorway, which leads into a light, open-plan 230,000sq ft space that for some reason reminds me of the British Library.

But rather than books, this place boasts an impressive collection of cool women. While I'm waiting for Marant, one effortlessly chic Parisian lady after another passes by. Skinny jeans, denim shirts, trainers or ankle boots. Nothing flash. Nothing, as the designer says later in her glorious Franco-English, that would 'make people peer at you in the street'. Or at least, if they did, it would only be to admire your understated zhuzh-iness.

When Marant has tamed the sweater (she's also wearing laced leather trousers and worn-down ankle boots), we race upstairs to the mezzanine.

Her greying hair is pulled back and she's wearing no make-up. Her face is almost ferociously expressive; when she grimaces or laughs her face creases into great concertina lines that stay lightly etched in her olive skin. She's gorgeous.

She asks someone for a coffee and, crossing one leg over another, rolls a cigarette. Really, she says, the demands of her job are so great that she should be like an élite sportive; early nights, swimming, yoga, no smoking. But the autumn/winter 2013 collection that she is preparing has caused her a lot of stress, plus, she's just shipped spring/summer 2013 into shops - a joyful collection inspired by Elvis and Hawaii - and she 'cannot refrain from doing things that are not good for me,' she says. 'But I'm working on myself'.

Marant after her AW 2012 show.

When we meet the catwalk show at which she will present the autumn collection is just a month away and she is 'exhausted, excited, because it's coming, frustrated because it's never what you want'. But her hand is itchy, 'and in France that's a sign that money's going to come'. She laughs, a surprisingly full, rather masculine guffaw. 'So I'm not very 'appy, but my hand says it's going to sell.'

It usually does. In the past decade Isabel Marant has gone from cult Parisian designer to international commercial big hitter. Worn by key influencers - Alexa Chung, Katie Holmes, Victoria Beckham, to name a few - she is a fashion editor's favourite, but there are many, many non-famous women who regularly welcome Marant's clothes into their wardrobes.

Once you've become familiar with her aesthetic her signature is instantly recognisable: a skinny, leggy silhouette: feminine but grounded by the masculine, a little bit rock 'n' roll, a little bit global traveller - she likes her ethnic craftmanship. Simple but, as she says, 'not very minimalist'. In short, it's how many women want to dress now and, as a result, her designs have been much emulated by other labels and the high street. Even if you've never heard of Isabel Marant you've probably worn something influenced by her.

'There's not so many brands you can dress up in every day,' Marant says, trying to explain her success. 'For when you have to work, you don't have a driver, when you have to run, bring your kids to school, look good at the office and just after go to dinner with friends. I mean, that's our life.'

She has always designed for herself, a strength in an industry where many womenswear creative directors are male. 'It's true that I am my own muse. I don't like this word but when I studied fashion at Studio Berçot [the Paris fashion college] the director said, "You shouldn't want others to wear things that you won't wear yourself," and that's something that never left me.'

In the past she has described her woman as someone who starts getting dressed up and then decides she can't be bothered and puts on her jeans instead; my kind of woman. 'Most of women are like this, come on,' she says. 'You want to look good but you don't want to spend too much time on it. I will always go to things that make me feel comfortable. Being a woman I've got a very honest eye on woman, I don't have any fantasy. Sometimes I can create things that are beautiful but I say, "When am I going to wear that?" Every morning when I open my cupboard I need some clothes, I don't need some clothes to go to awards or whatever, because it's once a year.'

Her personal staples are 'a pair of tight jeans, a pair of flat shoes and something that is a bit like a sweatshirt, a jumper or T-shirt. A good jacket, a good coat. I'm quite androgonystic. I'm very feminine but I always need to break it with something very masculine.'

For evening she'll just change her shoes to heels. She doesn't design eveningwear. In fact, she says sometimes she would prefer a world without it. 'It's so ugly when I see beautiful girls wearing these dresses because they have to. Sometimes they would look better wearing a well-cut trouser and a T-shirt. Much more chic than a… cake dress.'

She does a nice line in frank; none of that sitting on the fence that you so often find with the well-known. Disgust is a word that she employs a lot. She's disgusted that so many labels have copied her bestselling wedge trainers without attribution ('Everybody who has the wrong one looks quite bitchy, very vulgar, when mine are not at all'); she's disgusted by the impact on fashion of the digital age ('As soon as you have an idea it's copied and you don't like it in a minute'); and she's disgusted at the increased pressure on designers to provide two extra mid-season collections per year. 'I feel like a machine to vomit garments,' she says, rather vividly.

But given that she tells me she starts every collection from a feeling of 'disgust and frustration', perhaps this is a motivating state for her. 'I always think, "OK, again another collection. Why new clothes, when we have so many, I know we don't need it. What is going to be the difference?" It's always rejection, love and hate. Then I [go through] a psychological healing.' She laughs. 'From that, the excitation comes again. I go, "Yeah, that's a good idea, I must do it, I didn't do that properly before." Then it's a bit like a puzzle; there's a shoulder [shape] coming, a length is coming, a kind of fabric... I'm very instinctive, actually. I'm not very intellectual.'

As a child Isabel Marant didn't want to be a designer. Until she was 15, 'fashion was all that I hated'. Her mother was a successful model, but her father was keen to protect his daughter from the fashion industry, so the young Isabel 'didn't really understand' what she was doing: 'My mother was a kind of far-off angel.' Her parents separated when she was six and Marant lived with her father who remarried - 'a Caribbean woman, super-chic, like a model of Yves Saint Laurent in the 1970s. Marant was not impressed with her style. 'A lot of make-up. It was during the 1980s so it was really waaagh! And I was totally grunge. But even if it was too feminine for me, at least it printed in my brain this French chic. Very classic.'

At the other end of the style scale was her nanny, the final of her 'three mothers'. 'She was crazy. She was drinking, smoking and had ugly clothes,' she recalls, 'but she was wearing them in such a cool way that, finally, she was a kind of icon for me.'

Perhaps it was inevitable, then, that, aged 11, Marant was going to school 'dressed up in a very fancy way'. She made dresses out of her father's old silk paisley robes and wore them with his checked slippers. 'He had great cashmere sweaters and was quite tall and big, so on me they were looking super-nice.' She had a strong idea of how she wanted to look? 'Oh, completely. I didn't want to look like the others and I hated myself so I was hiding my face with my hair. I was quite ugly.' She shrugs. 'I was not a pretty girl.'

'That's how you felt,' I say, feeling protective of the little Isabel.

'I was not,' she says, definitively. 'No, because I was sucking my thumb and my teeth were all like that [she juts them over her bottom lip]. My little brother was just super-cute and had a super-nice angel face when I was quite… I was always like that… [she frowns]. I was a bit like Patti Smith. I was raised in quite a chic area of Paris and they were all wearing pleated navy-blue skirts and for me that was not…' She gives up searching for the word. 'Impossible.' She laughs.

At 15 she started to notice that fashion was changing. 'When [Jean Paul] Gaultier started, Comme des Garçons, Yohji Yamamoto… they kicked out all those Thierry Mugler shoulders, Montana, YSL.' She went crazy for Vivienne Westwood. 'She had a shop in Paris and I was babysitting for hours just to be able to buy something there. I had this huge red-pink skirt with two big pockets that were hanging. It was the opposite of all that was fashion until then'.

In a DIY, grungy spirit she started making her own clothes. 'In France we have nice cloths, red stripes, to sweep the floor and also to sweep the dishes. I made tops with one at the front, one at the back, square, very Japanese - in a European, trashy way.' Her friends coveted them. 'I was kind of earning my living at 16 selling my rubbishy things and I thought, "Eh, interesting." I'd wanted to do economic studies but I discovered fashion.'

After college she tried working in other houses but found the experience frustrating - too big and noisy in that 1980s way she didn't like - and decided to go it alone, launching her label in 1994.

Although she started as small as can be with a fashion show in a squat and her friends as models, it never felt like a struggle, 'because it was never a dream for me to be super-successful, recognised by all the fashion girls. I'm always surprised that I reached such a top level because I never thought about that. I built my company step by step. I was very lucky that I had great people working for me who pushed me a lot. It's not really pleasing me to have my name big and to be "ISABEL MARANT!" That's not what pleases me about my work.' She grins.

Despite the international sales, the healthy turnover (€62 million in 2011, up 44 per cent compared with 2010), the two lines (Etoile Isabel Marant is her cheaper diffusion line) and the 13 international stores - including four in Paris and a British boutique on its way - it still isn't. Although she works ferociously during the week, every weekend for the past six years she and her partner, the accessories designer Jérôme Dreyfuss, and their nine-year-old son, Tal, have escaped to a cabin in a forest outside Paris with no electricity and no hot water.

'What I love about it is that finally you [realise] you don't need a lot to be happy in life. And that's very reassuring to me,' she says. 'You know when you have all that pressure, pressure, pressure and sometimes I'm crying because I cannot achieve things, but when I'm in my cabin in the forest I say, "Finally I'm so happy." I'm really somebody who believes less is more. I think we have too many things. It's just killing everyone.'

She and Dreyfuss, who have been together for 16 years but have not married because 'they don't have time' (she says they will do it when they retire) are very similar in their outlook. 'I think we found each other because we share that same point of view,' she says. 'We have the same taste, we admire the same painters, designers, singers or whatever… but fashion is not only our life. It's what we love and what we do well, but we have many other things beside this.'